1. Technical Field
The present invention is directed to the manufacture of epoxy based shape memory polymers (SMPs), their production and use. More particularly, the current invention comprises a reaction mixture of at least one monofunctional amine reagent with at least one cross linking multifunctional amine reagent which is then further mixed with at least multifunctional epoxide whereupon the resulting mixture is cured.
The present invention is specifically drawn toward applications which currently use epoxy resin. The need for an epoxy SMP that is obvious to those of skill in the art. The present invention is also drawn to a shape memory polymer thermosetting resin having compatibility with polymers employed in high temperature, high strength and high tolerance processes in manufacturing.
2. Background Art
Shape memory materials are materials capable of distortion above their glass transition temperatures (Tgs), storing such distortion at temperatures below their Tg as potential mechanical energy in the material, and release this energy when heated again to above the Tg, returning to their original “memory” shape.
The first materials known to have these properties were shape memory metal alloys (SMAs), including TiNi (Nitinol), CuZnAl, and FeNiAl alloys. These materials have been proposed for various uses, including vascular stents, medical guide wires, orthodontic wires, vibration dampers, pipe couplings, electrical connectors, thermostats, actuators, eyeglass frames, and brassiere underwires. With a temperature change of as little as 10° C., these alloys can exert a stress as large as 415 MPa when applied against a resistance to changing its shape from its deformed shape. However, these materials have not yet been widely used, in part because they are relatively expensive.
Shape memory polymers (SMPs) are being developed to replace or augment the use of SMAs, in part because the polymers are light weight, high in shape recovery ability, easy to manipulate, and economical as compared with SMAs. SMPs are materials capable of distortion above their glass transition temperature (Tg), storing such distortion at temperatures below their Tg as potential mechanical energy in the polymer, and release this energy when heated to temperatures above their Tg, returning to their original memory shape. When the polymer is heated to near its transition state it becomes soft and malleable and can be deformed under resistances of approximately 1 MPa modulus. When the temperature is decreased below its Tg, the deformed shape is fixed by the higher rigidity of the material at a lower temperature while, at the same time, the mechanical energy expended on the material during deformation will be stored. Thus, favorable properties for SMPs will closely link to the network architecture and to the sharpness of the transition separating the rigid and rubbery states.
Heretofore, numerous polymers have been found to have particularly attractive shape memory effects, most notably the polyurethanes, polynorbornene, styrene-butadiene copolymers, and cross-linked polyethylene.
In literature, SMPs are generally characterized as phase segregated linear block co-polymers having a hard segment and a soft segment, see for example, U.S. Pat. No. 6,720,402 issued to Langer and Lendlein on Apr. 13, 2004. As described in Langer, the hard segment is typically crystalline, with a defined melting point, and the soft segment is typically amorphous, with a defined glass transition temperature. In some embodiments, however, the hard segment is amorphous and has a glass transition temperature rather than a melting point. In other embodiments, the soft segment is crystalline and has a melting point rather than a glass transition temperature. The melting point or glass transition temperature of the soft segment is substantially less than the melting point or glass transition temperature of the hard segment. Examples of polymers used to prepare hard and soft segments of known SMPs include various polyacrylates, polyamides, polysiloxanes, polyurethanes, polyethers, polyether amides, polyurethane/ureas, polyether esters, and urethane/butadiene copolymers.
The limitations with these are other existing shape memory polymers lie in the thermal characteristics and tolerances of the material. The Tg of a material may be too low for the conditions in which the system will reside, leading to the material being incapable of activation. An example of such a situation is an environment with an ambient temperature exceeding the transition temperature of the SMP; such a climate would not allow the polymer to efficiently make use of its rigid phase. Additionally, current organic systems from which SMPs are synthesized are not capable of operating in adverse environments that degrade polymeric materials. An example of such an environment is low earth orbit, where intense radiation and highly reactive atomic oxygen destroy most organic materials.
Applications for a shape memory material capable of withstanding these harsh conditions as well as higher thermal loads include, but are not limited to; morphing aerospace structures and space compatible polymers capable of self-actuation and dampening.
As discussed in Langer, SMP can be reshaped and reformed multiple times without losing its mechanical or chemical properties. When the SMP described by Langer is heated above the melting point or glass transition temperature of the hard segment, the material can be shaped. This (original) shape can be memorized by cooling the SMP below the melting point or glass transition temperature of the hard segment. When the shaped SMP is cooled below the melting point or glass transition temperature of the soft segment while the shape is deformed, a new (temporary) shape is fixed. The original shape is recovered by heating the material above the melting point or glass transition temperature of the soft segment but below the melting point or glass transition temperature of the hard segment. The recovery of the original shape, which is induced by an increase in temperature, is called the thermal shape memory effect. Properties that describe the shape memory capabilities of a material are the shape recovery of the original shape and the shape fixity of the temporary shape.
Conventional shape memory polymers generally are segmented polyurethanes and have hard segments that include aromatic moieties. U.S. Pat. No. 5,145,935 to Hayashi, for example, discloses a shape memory polyurethane elastomer molded article formed from a polyurethane elastomer polymerized from of a dysfunctional diisocyanate, a difunctional polyol, and a difunctional chain extender.
Examples of additional polymers used to prepare hard and soft segments of known SMPs include various polyethers, polyacrylates, polyamides, polysiloxanes, polyurethanes, polyether amides, polyurethane/ureas, polyether esters, and urethane/butadiene copolymers. See, for example, U.S. Pat. No. 5,506,300 to Ward et al.; U.S. Pat. No. 5,145,935 to Hayashi; and U.S. Pat. No. 5,665,822 to Bitler et al.
Several physical properties of SMPs other than the ability to memorize shape are significantly altered in response to external changes in temperature and stress, particularly at the melting point or glass transition temperature of the soft segment. These properties include the elastic modulus, hardness, flexibility, vapor permeability, damping, index of refraction, and dielectric constant. The elastic modulus (the ratio of the stress in a body to the corresponding strain) of an SMP can change by a factor of up to 200 when heated above the melting point or glass transition temperature of the soft segment. Also, the hardness of the material changes dramatically when the soft segment is at or above its melting point or glass transition temperature. When the material is heated to a temperature above the melting point or glass transition temperature of the soft segment, the damping ability can be up to five times higher than a conventional rubber product. The material can readily recover to its original molded shape following numerous thermal cycles, and can be heated above the melting point of the hard segment and reshaped and cooled to fix a new original shape.
Recently, SMPs have been created using reactions of different polymers to eliminate the need for a hard and soft segment, creating instead, a single piece of SMP. The advantages of a polymer consisting of a single crosslinked network, instead of multiple networks are obvious to those of skill in the art. The presently disclosed invention uses this new method of creating SMPs. U.S. Pat. No. 6,759,481 discloses such a SMP using a reaction of styrene, a vinyl compound, a multifunctional crosslinking agent and an initiator to create a styrene based SMP.
The industrial use of SMPs has been limited because of their low transition temperatures. Epoxy resins are a unique class of material which possesses attractive thermal and mechanical properties. Epoxy resins polymerize thermally producing a highly dense crosslinked network. Typically these thermoset epoxy networks are rigid and have low strain capability. By altering this network system, it is possible to produce a lightly crosslinked network still possessing many of the original materials properties but with the functionality of a shape memory polymer. Currently there is no epoxy based SMP available.
High temperature, high toughness thermoset resins with shape memory characteristics are not currently available. Other high temperature, high toughness, thermoset resins do not have shape memory. Typically, epoxy resins do not exhibit the shape memory effect mentioned above. In order to exhibit this shape memory effect epoxy resins must be crosslinked in a manner different from normal epoxy resins. It is this new method of crosslinking epoxy resins that is highly sought after.